LLC resonant circuit is characteristic of high efficiency and larger ranges of input voltage and load, so that it becomes a better architecture for conversion circuit. However, if LLC resonant circuit is cooperated with the conventional diode rectification circuit for providing output, the rectification diode will almost cause half of the whole loss. Therefore, synchronous rectification technology is used for improving the efficiency, as shown in FIG. 1, which shows the LLC resonant converter employing synchronous rectification technology. In FIG. 1, the converter is connected with a power source 1 for obtaining an input power. The power source 1 is connected to a switch unit 2, which is controlled by a pulse width modification unit 21, so that the current period of the switch unit 2 can be adjusted. The other terminal of the switch unit 2 is connected to a resonant circuit 3, which is an LLC circuit architecture composed of a resonant capacitor 31, a first resonant inductor 32 and a second resonant inductor 33. After passing through the switch unit 2, the conducting period and the flow direction of the input power are adjusted, and then, through a gain modification provided by the resonant circuit 3, the input power is transmitted to a transformer 4 for providing energy to the secondary side of the transformer 4. The secondary side of the transformer 4 has two synchronous rectifiers (SR) 61, 62 and also two synchronous controllers 51, 52 for respectively detecting the polar variation of power so as to produce a driving pulse to drive the synchronous rectifiers 61, 62, thereby synchronizing the synchronous rectifiers 61, 62 and the secondary side current of the transformer 4. Although in the market there already has IC can control the rectifiers 61, 62 (such as the product IR 1167 produced by INTERNATIONAL RECTIFIER), the above described IR 1167 is still limited by the maximum working frequency of 500 kHz, the high cost and the requirement for synchronous rectifier parasitic inductor, which restrict the application thereof.
Currently, a current driven synchronous rectifier (CDSR) is developed for satisfying the operation requirement of LLC converter under high frequency. However, as increasing the output current, there will be two problems arisen at high frequency and high output current. One is the synchronous controllers 51, 52 for the two rectification loops (as Loop 1 and Loop 2 surrounded by dashed lines), as shown in FIG. 1, might become too sensitive to cause error, and the loop inductance formed by AC impedance and rectifier loop will have a significant rise so as to reduce efficiency. The other is both the efficiency and the energy density of CDSR will become low, so as to influence the performance of LLC converter negatively.